1. Field of the Invention
The present invention relates generally to solid-state imaging detectors of ionizing radiation and, in particular, to amorphous selenium radiation detectors having an ultra-fast photo response and ultra-high time resolution.
2. Description of the Related Art
Amorphous selenium (a-Se) was previously developed for photocopying machines. A-Se has been commercially revived as a direct x-ray photoconductor for Flat-Panel Detectors (FPDs) due to high x-ray sensitivity and uniform evaporation over a large area as a thick film. However, current direct conversion FPDs are limited by, inter alia, degradation of low-dose imaging performance due to electronic noise, because energy required to generate an electron-hole pair in a-Se is 50 eV at 10 V/micron. Although other photoconductive materials with higher conversion have been investigated, the other photoconductive materials suffer from charge trapping and manufacturing issues. Improved conversion of a-Se is possible by increasing the electric field above 30 V/micron, i.e., 30,000 V on a 1,000 micron layer. However, such electric field increase is extremely challenging for reliable detector construction and operation, and is virtually impractical.
High resistivity amorphous solids used as photoconductors, especially amorphous selenium, are of interest because the high resistivity amorphous solids are readily produced over a large area at substantially lower cost than grown crystalline solids.
However, amorphous solids, i.e., non-crystalline solids with disorder, have been ruled out as viable radiation imaging detectors in a photon-counting mode because of low temporal resolution due to low carrier mobilities and transit-time limited pulse response, and low conversion gain of high energy radiation to electric charge. Avalanche multiplication in selenium can be used to increase the electric charge gain. However, significant obstacles prevent practical implementation of a direct conversion a-Se layer with separate absorption and avalanche regions.
A-Se has approximately 90% detection efficiency in blue wavelength, making A-Se ideal for coupling to Time of Flight (TOF) specific scintillators for high-energy radiation detection. A drawback of a-Se is poor time-resolution and low mobility due to shallow traps, problems that conventional devices have not circumvented for TOF detectors.
Direct conversion x-ray Flat-Panel Imagers (FPIs) provide high resolution and high detection efficiency, and detectors based on active matrix Thin Film Transistor (TFT) array readout of amorphous selenium photoconductor have been commercialized for general radiographic as well as mammographic clinical applications. However, conventional systems have only shown continuous and stable avalanche multiplication in a-Se, a feature that enabled development of an optical camera one hundred times more sensitive than a state of the art Charge Coupled Device (CCD) camera. See, M. M. Wronski, et al., Med. Phys. 37, 4982 (2010); and K. Tanioka, J. Mater. Sci., Mater. Electron. 18, pp. 321-325 (2007).
Positron Emission Tomography (PET) is a nuclear medical imaging modality that produces three dimensional (3D) images to see functional processes in human body. PET is commonly used in clinical oncology for detecting cancer, and for clinical diagnosis of heart problems and brain disorders. After positron-emitting radionuclides are introduced into the body, the radionuclides decay with each annihilation emitting two photons in diametrically opposing directions. TOF PET systems detect these photons, use TOF information to determine if two registered photons are in time coincidence, in which case the registered photons belong to a same positron annihilation event, and use the arrival time difference to localize each annihilation event. Without TOF localization data, computationally expensive iterative reconstruction algorithms are used to estimate 3D distribution of events that provide the best match with the measured projection data. Localization accuracy Δx of a TOF PET is determined by time-resolution Δt of the radiation detector, according to Δx=cΔt/2, where c is the speed of light.
An ultimate TOF detector, i.e., a TOF detector having a Δt less than 10 picosecond (ps), has not been realized. Existing commercial systems utilize PhotoMultiplier Tubes (PMTs) based on a plano-concave photocathode, which only achieve a Δt of approximately 500 ps. Silicon PhotoMultipliers (SiPMs), which are based on Geiger mode operating avalanche photodiodes, have achieved a better Δt, i.e., SiPM Δt˜100 ps. However, conventional systems suffer from high cost of PMTs and other components, complicated plano-concave photocathode arrangements, poor photon detection efficiency, optical crosstalk, small area, and poor uniformity.